Contact PD/PI: Tinoco, Roberto PROJECT SUMMARY Diseases leading to antigen persistence as in chronic viral infection and cancer induce development of exhausted T cells (Texs) that have defective effector function. While Texs represent an altered state of differentiation in which cells become addicted to antigen for their survival, their functions can be reinvigorated after inhibitory checkpoint blockade, leading to antigen control in murine, primate, and cancer patients. While only a small subset of patients responds to checkpoint blockade, many have severe toxicities especially with checkpoint combination therapy. The fact that clinicians expect most patients to be non-responsive to current immunotherapies highlights the pressing need to identify, develop, and improve treatments that are efficient and safe for patients that presently have no treatment options. Our discovery that chronically infected PSGL-1 (P-selectin glycoprotein ligand-1)-deficient mice mounted a robust anti-viral T cell response that supported viral clearance and was linked to downregulation of multiple inhibitory receptors demonstrates that PSGL-1 represents a new target for immune modulation. Our conclusion is underscored by our finding of enhanced anti-tumor T cell responses in PSGL-1-deficient mice leading to PD-1 downregulation and tumor control in a melanoma model. Our findings identified PSGL-1 as a new inhibitory checkpoint that promotes the development of Texs during chronic viral infection and cancer. Our overall objective in this proposal is to determine how and when PSGL-1 signaling inhibits T cells during chronic antigen stimulation. Our central hypothesis is that PSGL-1 signaling modulates inhibitory mechanisms not only in T cells but also in other immune cells that contribute to T cell exhaustion. The rationale is that once we fully understand this pathway, we can develop effective therapeutics targeting PSGL-1 to reinvigorate Texs. The proposed research will provide a mechanistic analysis of when, where, and how the PSGL-1 inhibitory signaling pathway mediates Texs development. We will test our hypothesis by pursuing the following three specific aims 1) To determine the cellular and temporal requirement for PSGL-1 mediated T cell exhaustion 2) To determine the therapeutic potential of relieving PSGL-1 inhibition and 3) To identify molecular mechanisms by which PSGL-1 mediates inhibitory pathways in T cells. This contribution will be significant because understanding how this inhibitory checkpoint axis fundamentally functions has the potential to lead to the development of new therapeutic interventions targeting a new class of inhibitory receptors to modulate immune responses and help prevent and treat a wide range of diseases. Our studies are innovative as they may expand the development of a new class of drugs targeting adhesion receptor inhibitory pathways to reverse T cell exhaustion. Understanding the mechanisms by which PSGL-1 mediates inhibitory signals is anticipated to help develop new therapies for diseases mediated by immune cell dysfunction. Project Summary/Abstract Page 6